The present invention relates to a cathode of an apparatus which observes the surface condition with an electron beam as represented by a scanning tunneling microscope (STM).
In observing the surface condition of a sample by flowing a tunnel current by placing a cathode tip very close to the sample that serves as an anode, it is accepted practice to provide the cathode tip with a piezoelectric material such as Pb(Zr, Ti)O.sub.3 (called PZT) and to move the cathode in very small amounts responsive to the change of the tunnel current in order to maintain constant the distance between the cathode and the anode as has been described, for example, in Drake et al., Rev. Sci. Instrum. 57(3), March 1986, pp. 441-445 and McCord et al., J. Vac. Sci. Technol. B4(1), January/February 1986, pp. 86-88. The scanning tunneling microscope employing a piezoelectric material has also been disclosed in Binnig et al., Surface Science 126(1983), pp. 236-244.
In general, a piezoelectric element of the stacked type has been used for coarse motion and a piezoelectric element of the single type has been used for fine motion. The piezoelectric material, however, consists of a sintered body which is a ferroelectric material having spontaneous polarization, so that it exhibits hysteresis and creeping characteristics in the strain thereof when a voltage is applied thereto. Therefore, the cathode must be driven with a frequency of as low as 1 KHz or lower. Because of this reason, it was so far impossible to measure the surface of sample at high speeds with a frequency of higher than 1 MHz.
The distance between the sample and the cathode tip undergoes the change due to expansion or contraction of the cathode tip that stems from the temperature change. Namely, error develops in the measurement. In order to prevent the development of error, it is desired to increase the scanning speed of the cathode tip. When it is required to grasp the surface condition of the sample in the form of signals of a high frequency, furthermore, the object cannot be sufficiently achieved with the traditionally employed frequency of lower than 1 KHz.